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According to the World Health Organisation, hepatitis C virus (HCV) infects approximately 170 to 200 million people worldwide. Whilst governments have sought to raise awareness on how HCV is transmitted through infected needles and body fluids and have implemented prevention programs, the incidence of HCV infection continues to increase. Approximately 80% of those who are infected with HCV remain carriers of the virus. In Australia about 16,000 new cases of HCV infection are reported each year, the new infections being most prevalent amongst injection drug users. HCV is the most common blood-borne viral infection, causing the death or morbidity of a substantial proportion of the population.
HCV is known to infect the liver and certain immune cells. As a result, HCV leads to serious liver diseases such as fibrosis, cirrhosis, steatosis and hepatocellular carcinoma (liver cancer) more frequently than other forms of hepatitis. HCV is a leading cause of cancer in liver transplant recipients. It is generally considered that the acute phase of the infection is often unrecognized due to the sub-clinical nature of the infection, and 80% of individuals progress to a chronic condition. Chronic infection is a result of the immune system's failure to generate a sterilizing immune response against the virus. There are six main HCV genotypes (1 to 6) and various subtypes (a, b, c, etc.). Currently, there is no vaccine for HCV and the only available treatment for HCV infection is anti-viral drugs. The general idea behind anti-viral drug design is to identify critical viral proteins, or parts of proteins, that can be disabled or inhibited. A standard treatment of choice for patients suffering moderate or severe fibrosis includes a combination of alpha-interferon and ribavirin. The antiviral effects of combination alpha-interferon and ribavirin therapy cause a rapid decrease in HCV levels in the blood, even after a single dose. Conventional alpha-interferon treatment for HCV however suffers several drawbacks. For example, (i) when alpha-interferon treatment is stopped after a few weeks or months of treatment, the viral load is rapidly re-established; (ii) treatment with alpha-interferon/ribavirin is associated with severe side effects, including flu-like symptoms, reduced red or white cell counts, suppression of bone marrow cells, neuropsychiatric effects, particularly depression and anemia; (iii) effective treatment requires patient adherence to a frequent dosing regimen since alpha-interferon is absorbed and eliminated from the body rapidly; and (iv) high cost of such treatments. Efficacy of the treatment varies with genotype and viral clearance is only obtained for approximately half of patients with genotype 1 or 4.
Some of the above drawbacks, referring particularly to item (iii) above, have been addressed by subjecting alpha-interferon to ‘pegylation’ in which polyethylene glycol molecules are attached to the interferon. The administration of pegylated interferon in combination with ribavirin increases the half-life of interferon and has the advantage of decreasing the frequency of dosing, hence improving patient compliance. Such treatment, however, has proven to be efficacious in less than 50% of treated patients. Given the increasing number of chronic sufferers of HCV, there is a need to develop a vaccine for both prophylactic and therapeutic purposes.
An essential component of all vaccines is the induction of virus neutralizing antibodies. In the case of HCV, the HCV glycoprotein E2 is the major target of the virus neutralizing antibody response. Neutralizing antibodies have been shown to be important for the clearance of HCV in animal models of HCV infection and in humans (Angus and Patel, 2011, Vanwolleghem et al., 2008, Law et al., 2008, Pestka et al., 2007). As HCV is a highly mutable virus, it is desirable that vaccines to prevent infection with HCV elicit neutralizing antibodies that are able to recognise the broad diversity of genotypes and subtypes of HCV.
The major cellular receptor for HCV is CD81 (Pileri et al., 1998). CD81 is required for entry into liver cells by all strains of HCV (Koutsoudakis et al., 2007, Zhang et al., 2004. McKeating et al., 2004, Bartosch et al., 2003. Pileri et al., 1998). Antibodies with the ability to prevent the interaction of viral particles with CD81 can be neutralizing. (Mancini et al., 2009, Law et al., 2008). High titres of E2-CD81 inhibition antibodies have been correlated with protection from HCV in vaccine studies (Youn et al., 2005) (Stamataki et al., 2008).
However, whilst E2-CD81 inhibition antibodies have been correlated with better protection against HCV in chimpanzees, they are not the only potential mechanism of neutralization. In other viral systems neutralization can be afforded through antibodies directed to the fusion loop (Sultana et al., 2009, Stiasny et al., 2006), and coreceptor interactions (Blish et al., 2008, Lusso et al. 2005). Furthermore, cross-linking of epitopes on the surface of flaviviral particles has been described as a major form of neutralization (Kaufmann et al., 2010).
The HCV glycoprotein E2 contains a discrete receptor binding domain (RBD) spanned by amino acid residues 384-661. The RBD may extend beyond residue 661 to the C-terminal boundary of the ectodomain of E2. Heterologous expression of the E2 RBD results in the secretion of a soluble protein that retains the ability to bind CD81. Within the RBD are three variable regions termed hypervariable region1, hypervariable region 2 (HVR2) and the intergenotypic variable region (igVR). These three variable regions reside outside the core domain of the glycoprotein and do not directly participate in the formation of the CD81 binding site of E2 (McCaffrey et al., 2007). Deletion of the three variable regions from the RBD results in the expression of a soluble form of the glycoprotein that is recognised by conformation dependent antibodies and retains wild-type (WT) levels of CD81 binding. This minimised form of the core domain of E2 is termed Δ123 E2661.
Given the drawbacks of current and experimental therapies for the treatment or prevention of HCV infection, there is an urgent unmet need to provide compositions capable of engendering an effective immune response in the treatment and prevention of HCV infection and able to generate diagnostic agents to detect HCV infection and for use in monitoring anti-HCV therapy.